The present invention relates to transition metal based polymerisation catalysts and to their use in the polymerisation and copolymerisation of olefins.
The use of certain transition metal compounds to polymerise 1-olefins, for example, ethylene, is well established in the prior art. The use of Ziegler-Natta catalysts, for example, those catalysts produced by activating titanium halides with organometallic compounds such as triethylaluminium, is fundamental to many commercial processes for manufacturing polyolefins. Over the last twenty or thirty years, advances in the technology have led to the development of Ziegler-Natta catalysts which have such high activities that olefin polymers and copolymers containing very low concentrations of residual catalyst can be produced directly in commercial polymerisation processes. The quantities of residual catalyst remaining in the produced polymer are so small as to render unnecessary their separation and removal for most commercial applications. Such processes can be operated by polymerising the monomers in the gas phase, or in solution or in suspension in a liquid hydrocarbon diluent. Polymerisation of the monomers can be carried out in the gas phase (the xe2x80x9cgas phase processxe2x80x9d), for example by fluidising under polymerisation conditions a bed comprising the target polyolefin powder and particles of the desired catalyst using a fluidising gas stream comprising the gaseous monomer. In the so-called xe2x80x9csolution processxe2x80x9d the (co)polymerisation is conducted by introducing the monomer into a solution or suspension of the catalyst in a liquid hydrocarbon diluent under conditions of temperature and pressure such that the produced polyolefin forms as a solution in the hydrocarbon diluent. In the xe2x80x9cslurry processxe2x80x9d the temperature, pressure and choice of diluent are such that the produced polymer forms as a suspension in the liquid hydrocarbon diluent. These processes are generally operated at relatively low pressures (for example 10-50 bar) and low temperature (for example 50 to 150xc2x0 C.).
Commodity polyethylenes are commercially produced in a variety of different types and grades. Homopolymerisation of ethylene with transition metal based catalysts leads to the production of so-called xe2x80x9chigh densityxe2x80x9d grades of polyethylene. These polymers have relatively high stiffness and are useful for making articles where inherent rigidity is required. Copolymerisation of ethylene with higher 1-olefins (eg butene, hexene or octene) is employed commercially to provide a wide variety of copolymers differing in density and in other important physical properties. Particularly important copolymers made by copolymerising ethylene with higher 1-olefins using transition metal based catalysts are the copolymers having a density in the range of 0.91 to 0.93. These copolymers which are generally referred to in the art as xe2x80x9clinear low density polyethylenexe2x80x9d are in many respects similar to the so called xe2x80x9clow densityxe2x80x9d polyethylene produced by the high pressure free radical catalysed polymerisation of ethylene. Such polymers and copolymers are used extensively in the manufacture of flexible blown film.
In recent years the use of certain metallocene catalysts (for example biscyclopentadienylzirconiumdichloride activated with alumoxane) has provided catalysts with potentially high activity. However, metallocene catalysts of this type suffer from a number of disadvantages, for example, high sensitivity to impurities when used with commercially available monomers, diluents and process gas streams, the need to use large quantities of expensive alumoxanes to achieve high activity, and difficulties in putting the catalyst on to a suitable support.
Patent Application WO98/27124 published on Jun. 25, 1998 discloses that ethylene may be polymerised by contacting it with certain iron or cobalt complexes of selected 2,6-pyridinecarboxaldehydebis(imines) and 2,6-diacylpyridinebis(imines).
An object of the present invention is to provide a novel catalyst suitable for polymerising monomers, for example, olefins, and especially for polymerising ethylene alone or for copolymerising ethylene with higher 1-olefins. A further object of the invention is to provide an improved process for the polymerisation of olefins, especially of ethylene alone or the copolymerisation of ethylene with higher 1-olefins to provide homopolymers and copolymers having controllable molecular weights. For example, using the catalysts of the present invention there can be made a wide variety of polyolefins such as, for example, liquid polyolefins, oligomers, resinous or tacky polyolefins, solid polyolefins suitable for making flexible film and solid polyolefins having high stiffness.
The present invention provides a polymerisation catalyst comprising (1) a nitrogen-containing iron compound having the following Formula Z, 
and (2) an activating quantity of an activator compound selected from organoaluminium compounds and hydrocarbylboron compounds wherein X represents an atom or group covalently or tonically bonded to the Fe and b is the valency of the atom or group X, characterised in that the catalyst is supported on (3) a solid particulate support material.
Each of the nitrogen atoms in the compound of Formula Z is coordinated to the Fe atom by a xe2x80x9cdativexe2x80x9d bond, ie a bond formed by donation of a lone pair of electrons from the nitrogen atom. The remaining bonds on each nitrogen atom are covalent bonds formed by electron sharing between the nitrogen atoms and the organic ligand.
The atom or group represented by X in the compound of Formula Z is preferably selected from halide, sulphate, nitrate, thiolate, thiocarboxylate, BF4xe2x88x92, PF6xe2x88x92, hydride, hydrocarbyloxide, carboxylate, hydrocarbyl, substituted hydrocarbyl and heterohydrocarbyl. Examples of such atoms or groups are chloride, bromide, iodide, methyl, ethyl, propyl, butyl, octyl, decyl, phenyl, benzyl, methoxide, ethoxide, isopropoxide, tosylate, triflate, formate, acetate, phenoxide and benzoate.
Examples of the preferred atom or group X in the compounds of Formula Z are halide, for example, chloride, bromide; iodide; hydride; hydrocarbyloxide, for example, methoxide, ethoxide, isopropoxide, phenoxide; carboxylate, for example, formate, acetate, benzoate; hydrocarbyl, for example, methyl, ethyl, propyl, butyl, octyl, decyl, phenyl, benzyl; substituted hydrocarbyl; heterohydrocarbyl; tosylate; and triflate. Most preferably X is selected from halide, hydride and hydrocarbyl. Chloride is particularly preferred.
The Formula Z compound is preferably 2,6-diacetylpyridinebis(2,4,6 trimethyl anil)FeCl2.
The activator compound for the catalyst of the present invention is suitably selected from organoaluminium compounds and hydrocarbylboron compounds. Suitable organoaluminium compounds include trialkyaluminium compounds, for example, trimethylaluminium, triethylalumninium, tributylaluminium, tri-n-octylaluminium, ethylaluminium dichloride, diethylaluminium chloride and alumoxanes. Alumoxanes are well known in the art as typically the oligomeric compounds which can be prepared by the controlled addition of water to an alylaluminium compound, for example trimethylaluminium. Such compounds can be iinear, cyclic or mixtures thereof. Commercially available alumoxanes are generally believed to be mixtures of linear and cyclic compounds. The cyclic alumoxanes can be represented by the formula [R16AlO]s and the linear alumoxanes by the formula R17(R18AlO)s wherein s is a number from about 2 to 50, and wherein R16, R17, and R18 represent hydrocarbyl groups, preferably C1 to C6 alkyl groups, for example methyl, ethyl or butyl groups.
Examples of suitable hydrocarbylboron compounds are dimethylphenylammoniumtetra(phenyl)borate, trityltetra(phenyl)borate, triphenylboron, dimethylphenylammonium tetra(pentafluorophenyl)borate, sodium tetrakis[(bis-3,5-trifluoromethyl)phenyl]borate, H+(OEt2)[(bis-3,5-trifluoromethyl)phenyl]borate, trityltetra(pentafluorophenyl)borate and tris(pentafluorophenyl) boron.
In the preparation of the catalysts of the present invention the quantity of activating compound selected from organoaluminium compounds and hydrocarbylboron compounds to be employed is easily determined by simple testing, for example, by the preparation of small test samples which can be used to polymerise small quantities of the monomer(s) and thus to determine the activity of the produced catalyst. It is generally found that the quantity employed is sufficient to provide 0.1 to 20,000 atoms, preferably 1 to 2000 atoms of aluminium or boron per Fe atom in the compound of Formula Z.
The solid particulate support material employed in the present invention can be, for example, any organic or inorganic solid which does not deleteriously affect the catalyst properties. The support can be, for example, an inorganic oxide, hydroxide or salt, for example, silica, alumina, silica-alumina, zirconia, magnesia (magnesium oxide), magnesium chloride, pumice, talc, kieselguhr, calcium carbonate, calcium sulphate; or an organic polymer or prepolymer, for example polyethylene, polystyrene, or poly(aminostyrene). Preferred supports are silica, alumina, zirconia, talc, kieselguhr, or magnesia. Particularly preferred are, for example, silica, alumina, or zirconia, or a polymer or prepolymer, for example polyethylene or polystyrene.
The support material is preferably free from absorbed water or other materials which might deleteriously affect the performance of the catalyst of the present invention.
The particle size of the support material is suitable in the range 5 to 500 microns, preferably in the range 20 to 200 microns.
If desired the catalysts can be formed in situ in the presence of the support material, or the support material can be pre-impregnated or premixed, simultaneously or sequentially, with one or more of the catalyst components. If desired, the support material itself can be a heterogeneous catalyst, for example, a Ziegler Natta catalyst supported on a magnesium halide, a Phillips type (eg chromium oxide/silica) supported catalyst or a supported metallocene catalyst. Formation of the supported catalyst can be achieved for example by treating the compound of Formula Z with alumoxane in a suitable inert diluent, for example a volatile hydrocarbon, slurrying a particulate support material with the product and evaporating the volatile diluent. The produced supported catalyst is preferably in the form of a free-flowing powder.
The quantity of support material employed in the catalyst of the present invention can vary widely, for example from 100,000 to 1 grams per gram of Fe present in the Formula Z compound.
The surface area of the support material (BET) employed in the present invention is preferably in the range 5 to 1000 m2 per gram, most preferably 50 to 500 m2 per gram.
The catalyst can be supported on the support material in any suitable manner, for example using conventional impregnation techniques. For example, the catalyst components can be dissolved or suspended in a suitable diluent or solvent and slurried with the support material. The support material thus impregnated with catalyst can then be separated from the solvent or diluent, for example, by filtration or evaporation techniques. Alternatively the supported catalyst can be stored in the presence of liquid diluent or solvent if desired.
When it is desired to produce the supported catalyst in the form of an essentially dry powder, the latter suitably contains not more than 30 weight %, preferably not more than 10 weight %, most preferably not more than 1 weight % of liquid diluent.
A further aspect of the present invention provides a polymerisation catalyst system comprising (1) a compound having the Formula Z, (2) an activating quantity of an activator compound selected from organoaluminium and hydrocarbylboron compounds, wherein the catalyst is supported on (3) a solid particulate support material, and further comprising (4) a neutral Lewis base.
In this further aspect of the present invention the preferences in relation to the activator compound, the nature of the support material and other characteristics of the catalyst of the present invention are the same as expressed above. Neutral Lewis bases are well known in the art of Ziegler-Natta catalyst polymerisation technology. Examples of classes of neutral Lewis bases suitably employed in the present invention are unsaturated hydrocarbons, for example, alkenes (other than 1-olefins) or alkynes, primary, secondary and tertiary amines, amides, phosphoramides, phosphines, phosphites, ethers, thioethers, nitrites, carbonyl compounds, for example, esters, ketones, aldehydes, carbon monoxide and carbon dioxide, sulphoxides, sulphones and boroxines. Although 1-olefins are capable of acting as neutral Lewis bases, for the purposes of the present invention they are regarded as monomer or comonomer 1-olefins and not as neutral Lewis bases per se. However, alkenes which are internal olefins, for example, 2-butene and cyclohexene are regarded as neutral Lewis bases in the present invention. Preferred Lewis bases are tertiary amines and aromatic esters, for example, dimethylaniline, diethylaniline, tributylamine, ethylbenzoate and benzylbenzoate. In this particular aspect of the present invention, components (1), (2), (3) and (4) of the catalyst system can be brought together simultaneously or in any desired order. However, if components (2) and (4) are compounds which interact together strongly, for example, form a stable compound together, it is preferred to bring together the components (1) and (2) in an initial step before adding component (4); or to bring together components (1) and (4) in an initial step before introducing component (2). The support material (3) can be introduced at any stage in the procedure. The quantities of components (1), (2) and (3) employed in the preparation of this catalyst system are suitably as described above. The quantity of the neutral Lewis Base [component (4)] is preferably such as to provide a ratio of component (1):component (4) in the range 100:1 to 1:1000, most preferably in the range 1:1 to 1:20. Components (1), (2), (3) and (4) of the catalyst system can brought together, for example, as the neat materials, as a suspension or solution of the materials in a suitable diluent or solvent (for example a liquid hydrocarbon), or, if at least one of the components is volatile, by utilising the vapour of that component. The components can be brought together at any desired temperature. Mixing the components together at room temperature is generally satisfactory. Heating to higher temperatures e.g. up to 120xc2x0 C. can be carried out if desired, e.g. to achieve better mixing of the components. It is preferred to carry out the bringing together of components (1), (2), (3) and (4) in an inert atmosphere (e.g. dry nitrogen) or in vacuo.
When it is desired to use a neutral Lewis base in the present invention, the supporting can be carried out, for example, by preforming the catalyst system comprising components (1), (2) and (4) and impregnating the support material preferably with a solution thereof, or by introducing to the support material one or more of the components simultaneously or sequentially. If desired the support material (3) itself can have the properties of a neutral Lewis base and can be employed as, or in place of, the optional component (4). An example of a support material having neutral Lewis base properties is poly(aminostyrene) or a copolymer of styrene and aminostyrene (ie vinylaniline).
The catalyst of the present invention can comprise, in addition to the compound of Formula Z, one or more other catalyst-forming transition metal compounds, for example one or more compounds having the general Formula B 
wherein
M is Fe[II], Fe[III], Co[I], Co[II], Co[III], Ru[II], Ru[III], Ru[IV], Mn[I], Mn[II], Mn[III] or Mn[IV]; X represents an atom or group covalently or ionically bonded to the transition metal M; T is the oxidation state of the transition metal M and b is the valency of the atom or group X; R1, R2, R3, R4, and R6 are independently selected from hydrogen, halogen, hydrocarbyl, substituted hydrocarbyl, heterohydrocarbyl or substituted heterohydrocarbyl;
and such that (1):
when M is Fe, Co or Ru, R5 and R7 are independently selected from hydrogen, halogen, hydrocarbyl, substituted hydrocarbyl, heterohydrocarbyl or substituted heterohydrocarbyl; and when any two or more of R1-R7 are hydrocarbyl, substituted hydrocarbyl, heterohydrocarbyl or substituted heterohydrocarbyl, said two or more can be linked to form one or more cyclic substituents,
or such that (2):
when M is Fe, Co, Mn or Ru, then R5 is represented by the group xe2x80x9cPxe2x80x9d and R7 is represented by the group xe2x80x9cQxe2x80x9d as follows: 
wherein R19 to R28 are independently selected from hydrogen, halogen, hydrocarbyl, substituted hydrocarbyl, heterohydrocarbyl or substituted heterohydrocarbyl; when any two or more of R1 to R4, R6 and R19 to R28 are hydrocarbyl, substituted hydrocarbyl, heterohydrocarbyl or substituted heterohydrocarbyl, said two or more can be linked to form one or more cyclic substituents; with the proviso that at least one of R19, R20, R21 and R22 is hydrocarbyl, substituted hydrocarbyl, heterohydrocarbyl or substituted heterohydrocarbyl when neither of the ring systems P and Q forms part of a polyaromatic fused-ring system,
or such that (3)
when M is Fe, Co, Mn or Ru, then R5 is a group having the formula xe2x80x94NR29R30 and R7 is a group having the formula xe2x80x94NR31R32, wherein R29 to R32 are independently selected from hydrogen, halogen, hydrocarbyl, substituted hydrocarbyl, heterohydrocarbyl or substituted heterohydrocarbyl; when any two or more of R1 to R4, R6 and R29 to R32 are hydrocarbyl, substituted hydrocarbyl, heterohydrocarbyl or substituted heterohydrocarbyl, said two or more can be linked to form one or more cyclic substituents; provided that said compound of Formula B is different from said compound of Formula Z.
Examples of suitable compounds of Formula B are
2,6-diacetylpyridinebis(2,6-diisopropylanil)FeCl2 
2,6-diacetylpyridine(2,6-diisopropylanil)MnCl2 
2,6-diacetylpyridine(2,6-diisopropylanil)CoCl2 
2,6-diacetylpyridinebis(2-tert.-butylanil)FeCl2 
2,6-diacetylpyridinebis(2,3-dimethylanil)FeCl2 
2,6-diacetylpyridinebis(2-methylanil)FeCl2 
2,6-diacetylpyridinebis(2,4-dimethylanil)FeCl2 
2,6-diacetylpyridinebis(2,6-dimethylanil)FeCl2 
2,6-dialdiminepyridinebis(2,6-dimethylanil)FeCl2 
2,6-dialdiminepyridinebis(2,6-diethylanil)FeCl2 
2,6-dialdiminepyridinebis(2,6-diisopropylanil)FeCl2 
2,6-dialdiminepyridinebis(1-naphthil)FeCl2 and
2,6bis(1,1-diphenylhydrazone)pyridine.FeCl2.
The catalysts of the present invention can also be employed in admixture with one or more other types of transition metal compounds or catalysts, for example, transition metal compounds of the type used in conventional Ziegler-Natta catalyst systems, metallocene-based catalysts, or heat activated supported chromium oxide catalysts (eg Phillips-type catalyst).
The present invention further provides a process for the polymerisation and copolymerisation of 1-olefins comprising contacting the monomeric olefin under polymerisation conditions with the polymerisation catalyst of the present invention.
Another embodiment of the present invention provides a process for the polymerisation and copolymerisation of 1-olefins comprising contacting the monomeric olefin under polymerisation conditions with the polymerisation catalyst of the present invention in the presence of hydrogen gas as a molecular weight modifier.
The polymerisation conditions can be, for example, solution phase, slurry phase or gas phase. If desired, the catalyst can be used to polymerise ethylene under high pressure/high temperature process conditions wherein the polymeric material forms as a melt in supercritical ethylene. Preferably the polymerisation is conducted under gas phase fluidised bed conditions.
Slurry phase polymerisation conditions or gas phase polymerisation conditions are particularly useful for the production. of high density grades of polyethylene. In these processes the polymerisation conditions can be batch, continuous or semi-continuous.
In the slurry phase polymerisation process the solid particles of supported catalyst are fed to a polymerisation zone either as dry powder or as a slurry in the polymerisation diluent. Preferably the particles are fed to a polymerisation zone as a suspension in the polymerisation diluent. The polymerisation zone can be, for example, an autoclave or similar reaction vessel, or a continuous loop reactor, e.g. of the type well-known in the manufacture of polyethylene by the Phillips Process. When the polymerisation process of the present invention is carried out under slurry conditions the polymerisation is preferably carried out at a temperature above 0xc2x0 C., most preferably above 15xc2x0 C. The polymerisation temperature is preferably maintained below the temperature at which the polymer commences to soften or sinter in the presence of the polymerisation diluent. If the temperature is allowed to go above the latter temperature, fouling of the reactor can occur. Adjustment of the polymerisation within these defined temperature ranges can provide a useful means of controlling the average molecular weight of the produced polymer.
In the process of the present invention employing hydrogen as molecular weight modifier, the quantity of hydrogen employed can vary widely. Generally, the higher the concentration of hydrogen employed, the lower the average molecular weight of the produced polymer. The polymerisation process employing hydrogen gas can be applied to control or reduce the average molecular weight of polymers or copolymers prepared using for example, gas phase, slurry phase or solution phase polymerisation conditions. The quantity of hydrogen gas to be employed to give the desired average molecular weight can be determined by simple xe2x80x9ctrial and errorxe2x80x9d polymerisation tests.
The polymerisation process of the present invention provides polymers and copolymers, especially ethylene polymers, at remarkably high productivity (based on the amount of polymer or copolymer produced per unit weight of the Formula Z compound employed in the catalyst system). This means that relatively very small quantities of the Formula Z compound are consumed in commercial processes using the process of the present invention. It also means that when the polymerisation process of the present invention is operated under polymer recovery conditions that do not employ a catalyst separation step, thus leaving the supported catalyst, or residues thereof, in the polymer (e.g. as occurs in most commercial slurry and gas phase polymerisation processes), the amount of residual Formula Z compound in the produced polymer can be very small. Experiments carried out with the supported catalyst of the present invention show that, for example, polymerisation of ethylene under slurry polymerisation conditions can provide a particulate polyethylene product containing catalyst so diluted by the produced polyethylene that the concentration of Fe therein falls to, for example, 1 ppm or less wherein xe2x80x9cppmxe2x80x9d is defined as parts by weight of Fe per million parts by weight of polymer. Thus polyethylene produced within a polymerisation reactor by the process of the present invention may contain catalyst diluted with the polyethylene to such an extent that the Fe content thereof is, for example, in the range of 1-0.0001 ppm, preferably 1-0.001 ppm.
Suitable monomers for use in the polymerisation process of the present invention are, for example, ethylene, propylene, butene, hexene, methyl methacrylate, methyl acrylate, butyl acrylate, acrylonitrile, vinyl acetate, and styrene. Preferred monomers for homopolymerisation processes are ethylene and propylene. The catalyst is especially useful for copolymerising ethylene with other 1-olefins such as propylene, 1-butene, 1-hexene, 4-methylpentene-1, and octene.
The catalyst of the present invention can also be used for copolymerising ethylene with other monomeric materials, for example, methyl methacrylate, methyl acrylate, butyl acrylate, acrylonitrile, vinyl acetate, and styrene.
The polymerisation conditions employed in the process of the present invention are preferably gas phase or slurry phase. Most preferably the polymerisation is conducted under gas phase fluidised bed conditions.
Methods for operating gas phase polymerisation processes are well known in the art. Such methods generally involve agitating (e.g. by stirring, vibrating or fluidising) a bed of catalyst, or a bed of the target polymer (i.e. polymer having the same or similar physical properties to that which it is desired to make in the polymerisation process) containing a catalyst, and feeding thereto a stream of monomer at least partially in the gaseous phase, under conditions such that at least part of the monomer polymerises in contact with the catalyst in the bed. The bed is generally cooled by the addition of cool gas (e.g. recycled gaseous monomer) and/or volatile liquid (e.g. a volatile inert hydrocarbon, or gaseous monomer which has been condensed to form a liquid). The polymer produced in, and isolated from, gas phase processes forms directly a solid in the polymerisation zone and is free from, or substantially free from liquid. As is well known to those skilled in the art, if any liquid is allowed to enter the polymerisation zone of a gas phase polymerisation process the quantity of liquid is small in relation to the quantity of polymer present in the polymerisation zone. This is in contrast to xe2x80x9csolution phasexe2x80x9d processes wherein the polymer is formed dissolved in a solvent, and xe2x80x9cslurry phasexe2x80x9d processes wherein the polymer forms as a suspension in a liquid diluent.
The gas phase process can be operated under batch, semi-batch, or so-called xe2x80x9ccontinuousxe2x80x9d conditions. It is preferred to operate under conditions such that monomer is continuously recycled to an agitated polymerisation zone containing polymerisation catalyst, make-up monomer being provided to replace polymerised monomer, and continuously or intermittently withdrawing produced polymer from the polymerisation zone at a rate comparable to the rate of formation of the polymer, fresh catalyst being added to the polymerisation zone to replace the catalyst withdrawn form the polymerisation zone with the produced polymer.
In the preferred embodiment of the gas phase polymerisation process of the present invention, the gas phase polymerisation conditions are preferably gas phase fluidised bed polymerisation conditions.
Methods for operating gas phase fluidised bed processes for making polyethylene and ethylene copolymers are well known in the art. The process can be operated, for example, in a vertical cylindrical reactor equipped with a perforated distribution plate to support the bed and to distribute the incoming fluidising gas stream through the bed. The fluidising gas circulating through the bed serves to remove the heat of polymerisation from the bed and to supply monomer for polymerisation in the bed. Thus the fluidising gas generally comprises the monomer(s) normally together with some inert gas (e.g. nitrogen) and optionally with hydrogen as molecular weight modifier. The hot fluidising gas emerging from the top of the bed is led optionally through a velocity reduction zone (this can be a cylindrical portion of the reactor having a wider diameter) and, if desired, a cyclone and or filters to disentrain fine solid particles from the gas stream. The hot gas is then led to a heat exchanger to remove at least part of the heat of polymerisation. Catalyst is preferably fed continuously or at regular intervals to the bed. At start up of the process, the bed comprises fluidisable polymer which is preferably similar to the target polymer. Polymer is produced continuously within the bed by the polymerisation of the monomer(s). Preferably means are provided to discharge polymer from the bed continuously or at regular intervals to maintain the fluidised bed at the desired height. The process is generally operated at relatively low pressure, for example, at 10 to 50 bars, and at temperatures for example, between 50 and 120xc2x0 C. The temperature of the bed is maintained below the sintering temperature of the fluidised polymer to avoid problems of agglomeration.
In the gas phase fluidised bed process for polymerisation of olefins the heat evolved by the exothermic polymerisation reaction is normally removed from the polymerisation zone (i.e. the fluidised bed) by means of the fluidising gas stream as described above. The hot reactor gas emerging from the top of the bed is led through one or more heat exchangers wherein the gas is cooled. The cooled reactor gas, together with any make-up gas, is then recycled to the base of the bed. In the gas phase fluidised bed polymerisation process of the present invention it is desirable to provide additional cooling of the bed (and thereby improve the space time yield of the process) by feeding a volatile liquid to the bed under conditions such that the liquid evaporates in the bed thereby absorbing additional heat of polymerisation from the bed by the xe2x80x9clatent heat of evaporationxe2x80x9d effect. When the hot recycle gas from the bed enters the, heat exchanger, the volatile liquid can condense out. In one embodiment of the present invention the volatile liquid is separated from the recycle gas and reintroduced separately into the bed. Thus, for example, the volatile liquid can be separated and sprayed into the bed. In another embodiment of the present invention the volatile liquid is recycled to the bed with the recycle gas. Thus the volatile liquid can be condensed from the fluidising gas stream emerging from the reactor and can be recycled to the bed with recycle gas, or can be separated from the recycle gas and sprayed back into the bed.
The method of condensing liquid in the recycle gas stream and returning the mixture of gas and entrained liquid to the bed is described in EP-A-0089691 and EP-A-0241947. It is preferred to reintroduce the condensed liquid into the bed separate from the recycle gas using the process described in our U.S. Pat. No. 5,541,270, the teaching of which is hereby incorporated into this specification.
When using the catalysts of the present invention under gas phase polymerisation conditions, the catalyst, or one or more of the components employed to form the catalyst can, for example, be introduced into the polymerisation reaction zone in liquid form, for example, as a solution in an inert liquid diluent. Thus, for example, the transition metal component, or the activator component, or both of these components can be dissolved or slurried in a liquid diluent and fed to the polymerisation zone. Under these circumstances it is preferred the liquid containing the component(s) is sprayed as fine droplets into the polymerisation zone. The droplet diameter is preferably within the range 1 to 1000 microns. EP-A-0593083, the teaching of which is hereby incorporated into this specification, discloses a process for introducing a polymerisation catalyst into a gas phase polymerisation. The methods disclosed in EP-A-0593083 can be suitably employed in the polymerisation process of the present invention if desired.